1. Field of The Invention
This invention relates to a method and apparatus for uniformly distributing a downflowing liquid in a liquid-vapor contact tower. The invention is not limited to the instance where the vapor flows upwardly in the tower, i.e. countercurrent to the liquid; but rather the invention also includes those instances wherein the vapor flows downwardly, i.e. co-currently with the liquid in the tower. However, for clarity's sake the invention will be described as though the vapor is always rising to better contrast the vapor from the liquid which is always falling.
2. Definitions Used Herein
A. "Liquid Distribution Apparatus"
It is recognized that in this art field a distinction is often made between a liquid "distributor" and a liquid "redistributor" with the primary distinction being that a liquid distributor does not collect liquid droplets falling randomly from sources located above the distributor in the tower such as from tower packing while a liquid redistributor does. The liquid "distributor" is usually known as only being capable of distributing liquid which is fed directly to it while the liquid "redistributor" is known as being capable of doing this in addition to collecting maldistributed liquid falling from above and uniformly distributing both sources of liquid.
Those distinctions are of no consequence to the present invention inasmuch as it is suitable for both liquid "distributors" and liquid "redistributors". Therefore liquid distributors and liquid redistributors will be used interchangeably herein, and "liquid distributing apparatus" will be used as meaning either liquid distributors or liquid redistributors or both.
B. Trough-type Distributor
A trough-type distributor is one which employs a plurality of spaced troughs, having closed ends and which are usually parallel. Liquid is usually fed to these troughs from one or more so-called "parting boxes" or "splitters" located above the troughs. Located in the sides of such troughs are triangular or rectangular weirs. The bottoms of these weirs are usually located at the same horizontal height when the distributor is installed in a tower. Gas rising in the tower is allowed to pass between the spaced troughs while liquid falls from the weirs. A representative example of such weir-type distributors can be found in U.S. Pat. No. 3,937,769 which is incorporated herein by reference as if set forth at full length. In some instances the troughs have orifices cut in their bottoms. See British Pat. No. 1,364,649.
C. Plate-type Distributor
A plate-type distributor as used herein is one which collects the liquid on a horizontal plate in the tower. They have ducts extending upward from holes cut in the plate to allow rising gas to pass freely through the plate. The ducts are sealably attached to the plate so that liquid on the plate passing downward from the plate has to pass either through orifices cut in the plate or through weirs or orifices cut in the sides of the ducts.
3. Discussion of Prior Art
Liquid distributing apparatus are used for various purposes in variously named vapor-liquid contact towers. Examples of such towers are fractionating columns, rectifiers, strippers, absorbers, and the like. These towers are usually equipped with means for uniformly distributing a liquid to a region of extended surface contact. The region of extended surface contact is usually a region in the tower which is packed with the materials commonly used as packing for fractionating columns; e.g. Raschig rings, Lessing rings, Pall rings, cross-partition rings, single-, double- and triple spiral rings, Berl saddles, Intalox saddles, continuous wire, sponge wire, and the like.
The reason for using distribution and redistribution means in the above circumstances is to assure that uniform wetting of the packing by the draining liquid is achieved. This is done so as to achieve uniform contacting conditions between the descending liquid and ascending vapor. This type of contact enhances transfer of mass and heat between the liquid and vapor phases. Failure to wet the packing evenly results in unequal liquid mass flow density throughout its volume. Variations can range from completely dry areas to flooded areas, both conditions being detrimental to column functionality and, in cases of temperature-sensitive liquids, material decomposition. Solids formation and plugging can also occur in the low mass flow density "dry-spot" sectors.
Various types of apparatus for distributing and/or redistributing liquids in materials exchange columns already exist and are well known in the art. Such are the trough-type and plate-type distributors defined above. The principal function of these distributors is to uniformly distribute liquid draining through them onto a bed of column packing located below them while allowing the flow of vapors upwardly through them in a sufficiently free manner so as not to incur any significant pressure drop in this vapor. This liquid distribution is done for the purpose of having uniform, intimate, and efficient mixing of the liquid and vapors in the column packing.
These distributors are usually satisfactory for obtaining uniform distribution of liquid in the column packing, for large flow rates, e.g. flow rates greater than 2 gallons/minute/sq. foot. However, for low flow rates the known distributors have not been satisfactory. Particularly is this true in towers where the liquid to be distributed has a very small flow rate; e.g. from about 0.15 to about 0.8 gallons/minutes per square foot of horizontal cross-section of the tower as measured at the cross-section of the tower where the liquid distribution oocurs. One problem with some of the trough and plate-type distributors is that the openings in their bottoms frequently plug up with debris or sediment from the liquid. This, of course, causes maldistribution of the liquid.
A liquid maldistribution problem common to all these liquid distributors (or redistributors) is the tendency of the liquid passing through them to wet the surface of the outlet of the distributor, adhere to this surface in a smearing fashion, and follow that surface to some unpredictable point where it drips off in some significantly maldistributed manner. This effect becomes more pronounced the further from perfectly horizontal the bottoms of these distributors are. Virtually all liquid distributors are less than perfectly horizontal when installed, and tend to become further so as the tower shifts further from the true vertical due to foundation shifting and the like.
Trough-type and plate-type distributors which have triangular or rectangular weirs in their sides of ducts, respectively, have a further disadvantage in producing a uniform liquid distribution. This disadvantage is that such uniform liquid distribution is greatly reduced by variations in liquid head pressure from weir to weir. These pressure variations can be either flow induced or produced from misfabrication or poor installation, or the like.
It is a common practice to design distributors so that, when liquid is flowed through weirs in the sides of troughs in trough-type distributors, or through similar weirs in the sides of ducts in plate-type distributors, the level of the liquid is maintained at a level below the tops of the weirs. The problem with such designs, however, is that they cannot produce acceptably uniform flow distribution at low flow rates (less than 2.0 gal./min./sq. ft.). This problem arises from being unable to maintain the bottom of the weirs at exactly the same horizontal height. Such deviation produces different head pressures above the bottom of the weirs. Of course, different head pressures will give different liquid flow rates out of the same weir; and when many of the same type weirs are used, as they are in these liquid distributors, and when the individual weirs have different head pressures, then there occur different flow rates from the different weirs. And, of course, different flow rates mean different liquid distribution, i.e. undesirable non-uniform liquid distribution.
One feature about triangular or rectangular weirs which those skilled in the art have apparently failed to appreciate is the difference in changes in flow rates out of unsubmerged different shaped weirs when subjected to different heat pressures. The flow rate out of an unsubmerged triangular weir is proportional to the head pressure raised mathematically to about the 2 1/2 power. Thus a slight head pressure variation in such a distributor will cause a proportionally small head pressure difference above the many triangular weirs spread across the distributor; but it will greatly multiply the difference in flow coming from the different weirs, and thus will greatly multiply the liquid flow maldistribution. Rectangular unsubmerged weirs are not as sensitive to different head pressures caused by slight head variations as are unsubmerged triangular weirs, but deviations still produce a disproportional flow variation from design flow rate. Through rectangular unsubmerged weirs the flow rate is proportional to the head pressure raised to about the 1.5 power.
On the other hand, flow through submerged openings is proportional to only about the square root (1/2 power) of the pressure head. Thus using submerged openings with variable head pressures produces a flow rate difference and maldistribution difference, but these differences are greatly suppressed when compared to the differences produced by triangular and rectangular weirs.
The present invention takes advantage of the use of submerged openings or orifices as well as greatly diminishing liquid maldistribution by other means including "drip rods" which are described herein below.